1. Field of the Invention
The present invention relates to a multi-layer container in which delamination is suppressed and which is excellent in gas barrier property, a mold suitable for manufacturing the multi-layer container, and a manufacturing method for a multi-layer container using the mold.
2. Description of the Related Art
At present, a multi-layer container such as a plastic container mainly made of polyester such as polyethylene terephthalate (PET) is widely used for tea, a fruit juice beverage, a carbonated beverage, and the like. The percentage of a small plastic container in the plastic container is increasing year by year. A reduction in container size increases a surface area per unit volume. Hence, as the plastic container becomes smaller in size, the shelf life of contents tends to become shorter. Further, in recent years, the plastic container has been used for a container for beer, which is liable to be affected by oxygen and light, and a plastic bottle has also been used as a container for hot tea. Thus, the utilization range of the plastic container is on an increasing trend. Accordingly, there is a demand for an additional improvement in gas barrier property of the plastic container.
In order to deal with the above-mentioned demand, a multi-layer container or a blend container using a thermoplastic polyester resin and a gas barrier resin, or a barrier coated container obtained by subjecting a single-layer bottle made of a thermoplastic polyester resin to carbon coating, vapor deposition, or barrier resin coating have been developed as a method of imparting gas barrier property to a multi-layer container.
As for one example of the multi-layer container, a container obtained by using a thermoplastic polyester resin such as PET as a resin for forming each of an innermost layer and an outermost layer and using a thermoplastic resin having gas barrier property such as poly-m-xylylene adipamide (called polyamide MXD6) as a resin for forming an inner layer, and injecting those resins simultaneously or successively to fill a mold cavity, molding a preform (parison) structure having three layers or five layers, and further subjecting the resultant preform to biaxial stretch blow molding has been put in practical use.
In addition, a resin having an oxygen scavenging function of scavenging oxygen in a container while blocking oxygen from the outside of the container has been developed and applied to the multi-layer container. There has been developed, as an oxygen scavenging container, a multi-layer container using, in a gas barrier layer, a “polyamide MXD6 resin” (manufactured by MITSUBISHI GAS CHEMICAL COMPANY, INC.) composition mixed with a transition metal-based catalyst in view of oxygen absorption rate, transparency, strength, moldability, and the like.
The above-mentioned multi-layer container is utilized as a container for beer, tea, a carbonated beverage, or the like because of its satisfactory gas barrier property. The multi-layer container is used for those applications to achieve the quality control of contents and the improvement of shelf-life. However, there is a problem in that delamination occurs between different resins, for example, between each of an innermost layer and an outermost layer and a gas barrier layer, resulting in a decrease in commercial value.
As for a method of solving such problem, Patent Document 1 describes that delamination is improved by, during finally injecting a resin for forming each of an innermost layer and an outermost layer into a mold cavity, interposing a crude mixed resin between layers using a back flow control apparatus capable of allowing a predetermined amount of the resin to flow backward to the side of a gas barrier layer, thereby preparing a preform. Further, Patent Document 2 describes a method including subjecting a preform to heat shrinkage during secondary molding and then blowing the resultant under high pressure again.